1. Field of the Invention
The present invention is in the field of imaging devices. More particularly, the present invention relates to devices for receiving invisible infrared light from a scene, and for providing a visible-light image replicating the scene.
2. Related Technology
Night vision devices have been available for many years. One category of these conventional night vision devices uses image intensifier technology. This technology is effected using a device generally known as an image intensifier tube. The image intensifier tube is essentially a frequency-shifting and amplifying device receiving ambient light, which light may include visible light too dim to provide natural vision (i.e., so-called "Star Light" scopes), or invisible near-infrared light, in a first frequency band and responsively providing a greatly intensified visible image in a phosphorescent monochrome yellow-green light.
Such an image intensifier night vision device converts available low-intensity ambient light to a visible image which a human user of the device may use for surveillance or weapon aiming, for example, under lighting conditions of too dim to allow a scene to be viewed with the natural vision. These image intensifier night vision devices require some residual light, such as moon or star light, in which to operate. This light is generally rich in near-infrared radiation, which is invisible to the human eye. The present generation of night vision scopes use a photoelectrically responsive "window" referred to as a photocathode, which is responsive to the dim or invisible ambient light focused on this "window" from an invisible scene to provide a pattern of photo-electrons flowing as a space charge moving under the influence of an applied electrostatic field, and replicating the scene being viewed. This pattern of photo-electrons is provided to a microchannel plate, which amplifies the electron pattern to a much higher level. To accomplish this amplification at the microchannel plate, the pattern of photo-electrons is introduced into a multitude of small channels (or microchannels) which open onto the opposite surfaces of the plate. By the secondary emission of electrons from the interior surfaces of these channels a shower of electrons in a pattern corresponding to the low-level image is produced. The shower of electrons, at an intensity much above that produced by the photocathode, is then directed onto a phosphorescent screen, again by the application of an electrostatic field. The phosphors of the screen produce an image in visible light which replicates the low-level image.
Image intensifier tubes have evolved from the so-called "Generation I" tubes through the more recent "Generation III" tubes, which provide greater amplification of available light and greater sensitivity to infrared light somewhat deeper into the infrared portion of the spectrum. However, these image intensifier devices are limited with respect to the depth into the infrared portion of the spectrum to which they can operate.
Another category of conventional night vision device is represented by the cryogenically cooled focal plane array thermal imaging devices. These devices use a photoelectrically responsive detector which is cooled to a temperature in the cryogenic range to reduce unwanted thermal noise. The detector includes a plurality of detector elements, or "pixels", each of which provides an electrical signal indicative of the flux of infrared light falling on the detector element. Some such devices use a staring focal plane array; while others have a linear focal plane array of detector elements, and require the use of a scanner to sequentially move portions of the viewed scene across the detector. In either case, because the detector is cooled to cryogenic temperatures, it can proved an electrical response to invisible infrared light much deeper into the infrared part of the spectrum than is possible with the image intensifier devices. The electrical signal provided by such a detector must be processed and converted to a visible image. For this purpose, many such devices of this category have used cathode ray tubes, liquid crystal displays, and other such display technologies to provide a visible image to the user of the device.
A significant disadvantage of this category of night vision device is the requirement for cryogenic cooling of the detector. Early devices of this category used a Dewar vessel into which a supply of a cryogenic fluid (such a liquid nitrogen) had to be provided by the user of the device. The utility of such devices was severely limited by their requirement for occasional replenishment of the cryogenic coolant. Later devices of this type have used cryogenic cooling developed by reverse Sterling-cycle coolers. However, such coolers require a considerable amount of power, are not without their own maintenance and reliability problems, and are generally noisy.